There are several endocrine tumor markers that are routinely used in the diagnosis and management of various cancers. The current assays, however, typically take several hours to perform, which precludes their use in the intraoperative management of patients. The majority of the endocrine tumor markers have a half-life in the circulation of less than 5 minutes, thus making it feasible, if a rapid assay was available, to monitor the concentration of the hormones during surgery or during localization procedures. The primary indication for such assays would be in assessing the extent of residual tumor after surgery and in the localization of the tumors by selective venous or arterial sampling. A commercial rapid assay for monitoring parathyroid hormone (PTH) has recently become available. The goal of this study will be to assess the clinical utility of the rapid PTH assay in the management of patients with hyperparathyroidism. In addition, rapid immunoassays for ACTH and gastrin will be developed and their clinical utility will be determined for the treatment of patients with Cushing's disease and gastrinoma, respectively.